This invention relates generally to the field of Solid Freeform Fabrication (SFF), also known as Rapid Prototyping, which is characterized by a layer-by-layer build-up to produce three dimensional parts or objects. In particular, this invention relates to an SFF process and apparatus for producing three dimensional parts using layer-by-layer deposition of a powder directly onto a previously laid layer of powder, where the deposition is accomplished using electro-photographic powder deposition technology to create the three-dimensional part.
SFF is a new manufacturing technique which is characterized by a layer-by-layer build-up of matter to form three-dimensional objects, which makes it possible to create significantly more complex objects in one fabrication step than previously possible. In addition, due to the relatively simple process planning required, the potential has been demonstrated to automatically fabricate a part under computer control given a solid model of the part. SFF technology was first used to create prototypes of designs for visualization and testing purposes. The trend is to develop SFF methodologies which can be used to directly produce actual parts, tools or molds of precise dimension and optimum physical properties rather than rough prototypes.
Several different technologies for SFF have been developed in recent years. The technologies are referred to as stereolithography, 3-D printing, selective laser sintering, laminated object manufacturing, fused deposition modeling, ballistic particle manufacturing, shape deposition manufacturing and laser-engineered net-shaping. The earliest developed SFF technique is stereolithography, which builds a part by solidifying a liquid photopolymer using a laser beam. Parts are constructed layer-by-layer by hardening the photopolymer using a laser beam that is projected in the shape of the cross-section of the part. A disadvantage of stereolithography is that it is limited to parts which can be constructed of photopolymers. Extensions of this technique are being developed for use with ceramics and other materials.
Fused deposition modeling (FDM) involves depositing ABS plastic, wax, certain elastomers or similar material by melt extrusion through a nozzle. The environment is temperature controlled so that the deposition material solidifies quickly upon extrusion. To construct complex shapes with overhanging features, support structures must be utilized both in FDM and stereolithography. These support structures must be subsequently removed manually. Ballistic particle manufacturing (BPM) uses a piezo-electric jetting system to deposit droplets of molten thermoplastic to form an object, and this technique likewise requires the use of support structures. The BPM jet head is mounted on a 5-axis positioning mechanism and controlled by software.
Shape deposition manufacturing (SDM) integrates material deposition and material removal. Layers of part material are deposited and machined to net shape before additional material and further layers are deposited. Microcasting, a welding process, is used to deposit molten metal droplets for creating fully dense parts. In each layer, the part material is deposited in the shape of the part cross-section and the remaining area is covered using a support material which is etched away after the part is complete. For example, stainless steel parts may be manufactured by SDM using copper as the support material.
Laminated object manufacturing (LOM) builds parts by gluing foils or sheets of material on the top of one another. A laser beam is used to cut the sheet into the desired shape of the cross-section. The material is stored as rolls of sheet material which is unwound and routed over a platform on which the part is built. Sheet material is glued to the layers below by a heated roller. The laser beam then cuts the desired cross-section of the part. The material that is to be removed is cut into a cross-hatched pattern to facilitate removal.
Selective laser sintering (SLS) is a powder based process which requires no support structures to create complex shapes. A thin layer of powder is deposited in a workspace container and heated to just below its melting point. The powder is then fused together using a laser beam that traces the shape of the desired cross-section. The process is repeated by depositing successive layers of powder and fusing each layer. The area that is not sintered remains as a loose powder that can be easily removed after all the layers have been deposited and fused. Previously deposited powder provides the support for any overhanging features of the part geometry. Typically, the position of the laser beam is controlled by a scanning mirror and the powder is deposited in a cylindrical workspace which has a moving base or platform, which is lowered after each layer of powder is deposited. The powder is deposited in uniform layers using a powder leveling roller. Suitable powder materials include polycarbonates, investment casting wax, PVC, ABS plastic and nylon. A wider range of materials can be used with the SLS system over the other techniques discussed above. A drawback is that additional powder at the boundaries is often hardened and remains attached to the part, thereby requiring additional finishing steps to remove the unwanted material. Furthermore, an inert atmosphere is required, increasing the cost of the equipment. Toxic fumes may be emitted from the powder material during processing.
The 3-D printing process has powder deposited in layers and selectively joined by a binder material. Ink-jet printing technology is used to print the binder in the shape of the cross-section of the part on each layer of powder. The powder is deposited on a platform which is lowered after each layer is deposited. After the whole part has been printed, heat treatment is required to consolidate the part. Regions where the binder was not deposited remain as loose powder which is removed after the heat treatment. This technology can be used with a wide variety of materials, and is currently used mainly to make ceramic molds for metal casting.
Freeform powder molding (FPM) is a dual powder method, one of which forms the part and the other of which is a support powder. The part powder is shaped by mixing it with an aqueous carrier, pouring it into a mold and freezing the mixture, removing the mold and surrounding the frozen part with a support powder. The combination is then sintered to create the finished part. Another method similar to this is disclosed in U.S. Pat. No. 5,555,481 to Rock et al., where powder is deposited in a layer-by-layer fashion, one powder acting as a mold or support for the other powder. The deposition technique is very crude and precision objects cannot be formed by this technique. The powder is simply gravity fed onto the previously deposited layer.
Laser-engineered net shaping (LENS), also known as direct light fabrication (DLF) or direct metal fabrication since its main application is in the construction of metal parts, builds three dimensional parts by delivering metal powder into the path of a high power laser beam. A Nd:YAG laser is used to melt an area on a metal substrate while a nozzle delivers the powder to the molten weld pool. The nozzle is stationary and the build platform is translatable over the X/Y plane. The method produces a metal bead, with successive layers built by adding beads on top of the previously deposited bead to define the part.
It is an object of the present invention to provide a novel SFF technique and apparatus which improves over the known SFF processes by providing for deposition of successive powder layers directly onto the previously deposited powder layer, by lowering equipment and processing costs, by eliminating the need to melt or bind the part material into a lamina prior to its deposition onto the previously deposited layer, by utilizing a secondary support powder to support a primary part powder which fuses or sinters to form the part at a temperature below the melt or fusion temperature of the secondary powder so that the support powder does not consolidate and is therefore easy to remove, by providing a method which can be used with a wide variety of powders, including polymers, ceramics, metals, alloys and mixed composition materials, whether conducting, non-conducting, magnetic or non-magnetic, by providing a method which produces high density, high strength and high toughness parts, by providing a method which allows the layer-by-layer composition to be altered to produce, for example, advanced metal matrix composites, fiber reinforced composites, parts with composition gradients, and parts with embedded circuits and electronics, and by providing a method which can deposit very fine powder precisely to create parts that have relatively small tolerances to produce precision parts and to create very small objects.
These and other objects, as more fully revealed below, are accomplished by providing a powder based solid freeform fabrication process and the apparatus for performing the process, where powder is deposited directly onto a previously deposited powder layer in successive layers corresponding to cross-sections of the three dimensional part using an electro-photographic deposition technique by charging the previously deposited powder layer opposite to the charge of the powder on the photoreceptor such that the attractive force of the powder to the previously deposited layer is greater than the attractive force to the photoreceptor means, resulting in the transfer of the powder onto the previously deposited layer.